1. Field of the Invention
This invention relates in general to a fixing device and an image forming device, which can be used in a facsimile device, a printer, a copier, or a multi-function machine with the above functions.
2. Description of the Related Art
FIG. 7 is a side view of an inter structure of a facsimile device having a conventional fixing device and a conventional image forming device. The facsimile device comprises a main body 1, a process cartridge 2 comprising various process devices for performing image formation by an electronic photographic process is detachably mounted inside the main body 1, a photoreceptor 3 contained within the process cartridge 2, an optical writing device 4 that makes the photoreceptor 3 to scan by a modulated laser beam based on an image data, a paper cassette 5 for storing the recording paper, a paper-feeding roller 6 for feeding the recording paper from the paper cassette 5, a transferring roller 7 that is in contact with the photoreceptor 3 and is used for transferring the toner image formed on the photoreceptor 3 onto the recording paper, a fixing device 8 having a fixing roller 9 and a pressure roller 10, a paper-ejecting roller 11, a sealed sensor 12, and document stage 13 formed over the main body 1.
The document placed on the document stage 13 is passed through the sealed sensor 12 by a transport system, and then ejected to external. As the document passes through the sealed sensor 12, the image on the document is optically read by the sealed sensor 12. The image data, which is read by the sealed sensor 12 or input from an external source, is transmitted to the optical writing device 4. Based, on the image data, the optical writing device 4 emits a modulated laser beam to the surface of the uniformly charged photoreceptor 3, so that an electrostatic latent image is formed on the surface of the photoreceptor 3. Then, the toner is made to adhere to the electrostatic latent image to form a toner image, and the toner image is transferred onto the recording paper by the transferring roller 7. When the recording paper transported from the transferring roller 7 passes through nip portion between the fixing roller 9 and pressure roller 10 of the fixing device 8, the toner image on the recording paper is melted by pressing and heating so as to be fixed onto the recording paper. Thereafter, the recording paper is ejected to external through the paper-ejecting roller 11.
FIG. 8 is a side view of an inter structure of a conventional fixing device. The fixing device 8 further comprises a fixing heater 19 that is disposed in the fixing roller 9 and is used for heating the fixing roller 9, a temperature detecting device 20 for detecting the temperature of the fixing roller 9, a separating claw 14 for separating the recording paper attached on the fixing roller 9 from the fixing roller 9, a cleaning pad 15 that is in contact with the fixing roller is used for cleaning the fixing roller 9, a cleaning pad invasion regulating unit that is installed at the downstream side in the fixing roller 9's rotational direction with respect to the cleaning pad, and an entrance guiding plate 18 installed at the entrance of the recording paper in main body of the fixing device.
The recording paper where the toner image has been transferred enters the nip portion between the fixing roller 9 and the pressure roller 10 through the entrance guiding plate 18. After the toner image is fixed onto the recording paper by pressing and heating at the nip portion, the recording paper is ejected to the external through the paper-ejecting roller 11. When the recording paper passes through the nip portion, the toner stock on the fixing roller 9 is removed by the cleaning pad 15.
The surface temperature of the fixing roller 9 is detected by the temperature detecting device 20. According to a detected result of the temperature detecting device 20, a heating control of the fixing heater 19 is performed by a controlling device (not shown).
Conventionally, an infrared heater or a halogen heater is used as the fixing heater 19 in the fixing device 8, and a structure constituted of the fixing roller and the pressure roller pair is most common. The pressure roller is a structure that is made by forming silicon rubber on a core bar and then a mold-releasing layer (such as Teflon, registered Trademark) is formed on the silicon rubber, so that the heat capacity of the pressure roller becomes larger. At the beginning of heating, the pressure roller does not get warm easily, causing incomplete fixing problems. Furthermore, there is also a demerit that assembly parts have high cost. In addition, in order to assure a suitable nip width of the nip portion between the fixing roller and the pressure roller, a large pressing force (pressure) is required. Because of the pressure, stress will be imparted on the recording paper, causing fixing wrinkle problems.
Instead of the pressure roller, a nip portion is formed by pressing a sheet to be in contact with the fixing roller. For example, a fixing device, where an unfixed image is fixed onto the recording paper by making the recording paper to pass the nip portion, is well studied and researched. By the above structure, the sheet can be easily warmed up, and the nip width can be also assured.
However, when starting the fixing process, the surface temperature on the ends of the fixing roller is lower than that on the central portion because the heat is being conducted to the bearings. As a result, the fixing rate of the toner on the ends of the recording paper at the beginning is worse. Therefore, the effect of the fixing device using the sheet-shaped pressure unit cannot be achieved from using the pressure roller. Namely, to accelerate warming up easily at the beginning of the heating like in the case of the sheet-shaped pressure unit cannot be achieved by using the pressure roller.
To solve this problem, a method to increase the heat distribution at the ends is provided. However, this method arises a hot offset image when small size recording papers are continuously fed, or increases thermal damages at the bearings.